This invention is directed to blow molding systems. More specifically, it is directed to stretch blow molding systems that condition preforms such that the temperature distribution within the cross-section of each preform is optimized prior to blow molding operations.
By way of background, one of the most critical process variables when attempting to stretch blow mold certain materials is the temperature distribution of the preform as it is being blown. This variable often has a significant impact on the most important physical properties of the final blown article. Ideally, the preform wall cross-sectional temperature distribution should be optimized for blow molding operations. In addition, it is advantageous within an automated blow molding process if preforms that are moving through the system are consistent in temperature profile, one preform to the next. In more specific terms, it is advantageous for both the inside and outside surface temperature of the preforms to be equal to each other and at the preferred blow molding temperature of the material.
The following collection of U.S. patents, all of which are incorporated herein by this reference, describe the current state-of-the-art associated with the thermal conditioning of preform blanks prior to automated blow molding operations:                U.S. Pat. No. 4,079,104        U.S. Pat. No. 5,066,222        U.S. Pat. No. 5,322,651        U.S. Pat. No. 5,607,706        
These patents collectively teach the following: 1) the use of infrared radiation to raise the temperature of a series of dynamically conveyed preforms to a transition temperature wherein blow molding operations are enabled and optimized, 2) the qualitative time-varying temperature behavior of both the outside as well as the inside of a preform when it is subjected to infrared radiation from the outside of the preform, 3) the benefit of using forced air cooling on the outside of the preform to enable the inside preform surface temperature to “catch-up” with the outside temperature, 4) the benefit of thermally pre-conditioning preform blanks prior to the infrared radiation re-heat cycle to compensate for preform-to-preform ambient thermal differences, 5) the practice of using pyrometers to provide some degree of average outside surface temperature information for the purpose of process control, and 6) the benefit of rotating the preforms on their axis as infrared radiation is applied to more uniformly deliver thermal energy prior to blow molding operations.
However, there is a need in the field to accurately determine and control both the outside and inside surface temperature of preforms as such items enter the blow molding operation. The state-of-the-art implementations, as described in, for example, the noted patents, are limited to providing an averaged automated process control measurement on the outside surface temperature of the preform, exclusively. Process information related to critical control parameters (e,g., inside and outside preform temperature) has been limited in scope due to existing limitations in infrared temperature measurement technology and methodology.
For example, in existing state-of-the-art implementations, the outside surface temperatures of preforms are measured using a pyrometer. Pyrometers are well known in the art and can perform accurate quantitative temperature measurements of objects placed within their field of view (FOV). A significant limitation of pyrometers, though, is that their response time is relatively slow. To obtain an accurate temperature measurement using a pyrometer, the object under test needs to remain in the pyrometer's field of view (FOV) for a period of time ranging from hundreds of milliseconds to seconds. In a dynamic blow molding system, the rate at which preforms are transported through the system does not allow for the accurate temperature measurement of any one preform using a pyrometer. Rather, state-of-the-art implementations use pyrometers to obtain an average surface temperature of the last several preforms passed through the system. One limitation of this sampling scheme is that significant preform to preform temperature variations, if present, are averaged out and go undetected by the process control apparatus deployed in prior art machines.
At the speeds at which preforms are transported through existing state-of-the-art machines, temperature measurement must be made within a couple of milliseconds—before the part moves out of the sensing device's field of view (FOV). This time period is much too short to allow pyrometers to make an accurate temperature measurement. Direct contact methods for measuring temperatures, such as thermocouples, are impractical because of speed limitations and because such devices may cause damage to the hot pliable preform. If such damage occurred, it would undesirably render the process a high maintenance endeavor.
An additional and perhaps more severe limitation related to the slow response time of pyrometers is that that there has been no direct practical manner discovered by which a reasonable and accurate measurement of the inside surface temperature of moving preforms can be obtained using these devices. The inside surface temperature of a preform 10 can only be directly measured through the open end of the preform. Indirect methods, using complex thermal conduction equations and multiple, time spaced pyrometers, have been theorized but have not been implemented into factory production systems. The difficulty of implementing, maintaining, and calibrating such systems is a serious drawback. Further, such systems only predict from indirect measurements—which allows uncontrolled environmental and other variables to adversely affect the temperature estimates and create inaccuracy and mistrust of such estimates.
The present invention, however, overcomes these difficulties and others.